1. Field of the Invention
This invention relates to x-ray imaging systems. In a primary application the invention relates to obtaining cross-sectional x-ray images which are free of spectral-shift artifacts and providing information about the materials in the cross section. In another application the invention relates to defining the materials in a projection radiograph.
2. Description of Prior Art
A number of computerized tomography instruments have recently been introduced which produce x-ray cross-sectional images of the human anatomy. This is accomplished by measuring the x-ray projections at a number of different angles and using various mathematical techniques to reconstruct the three-dimensional information. The system of the EMI brain scanner is described in papers by J. Ambrose and G. N. Hounsfield in the British Journal of Radiology, vol. 46, 1973, on pp. 1023-47 and 1016-1022.
One of the biggest sources of inaccuracy of these instruments is the spectral shift of the x-ray beam energy as it traverses the various materials in the cross-sectional slice. The log of the measured transmitted intensity should represent the sum or line integral of the linear attenuation coefficients along the x-ray beam. This will be the case if a monoenergetic source is used. These sources, however, have insufficient strength to provide a complete scan in a reasonable time interval. The use of broadband of polychromatic x-ray sources, which have sufficient strength, results in various non-linear artifacts since the attenuation coefficients are a function of energy. As the x-ray beam goes through different amounts and types of material, it has a different emerging energy spectrum which results in different measured attenuation coefficients. This is described in, "An Evaluation of the Quantitative and Radiation Features of a Scanning X-Ray Transverse Axial Tomograph," by E. C. McCullough, et. al., in Radiology, vol. 111, June 1974, on pages 709-715.
In an attempt to minimize this problem most instruments have done one or both of two remedies; the use of path-length compensators, and the use of relatively high x-ray energies. Unfortunately, these remedies are only partial cures to the artifact problem. In addition, the use of pathlength compensators increases the radiation to the patient. The use of relatively high x-ray energies results in the loss of important information about the photo-electric component of the attenuation coefficient which helps to distinguish various types of tissue.
When a path-length compensator is used, with its associated increased radiation dose, the spectral-shift artifact can be corrected if the object being studied consists solely of two materials, such as a specific type of bone and soft tissue. Some instruments use corrections of this type. This correction breaks down, however, due to the wide variations in the types of bone and soft tissue.
A system for providing a general correction of this problem is described in U.S. Pat. No 3,965,358 issued to A. Macovski entitled, "Cross-Sectional Imaging System Using a Polychromatic X-Ray Source." In this patent a number of spectral measurements are taken of the transmitted x-ray beam. These are processed to produce a cross-sectional image which is free of spectral-shift artifacts. In addition, rather than producing a single-component image as is done in existing instruments, a number of images are obtained indicative of the specific materials in the cross-section.
This patent failed to recognize, however, that, to a high degree of accuracy, the linear attenuation coefficient of most materials found in the body can be decomposed into a photoelectric component which is strongly dependent on the atomic number and a Compton-scattering component which is primarily dependent on density. These components can be reconstructed from relatively simple low-energy and high-energy transmission measurements using non-linear processing. Thus the spectral-shift artifact is removed and each material in the cross section is defined in terms of its average atomic number and its density. This system is described in a paper authored by the inventors, R. E. Alvarez A. Macovski, entitled, "Utilization of Simple Energy Spectrum Measurements In X-Ray Computerized Tomography," published in the Proceedings of the Conference on Image Processing for 2D and 3D Reconstruction from Projections, Aug. 4, 1976.
The same general technique can be used in conventional projection radiography for providing additional information about the materials in the object being studied. In projection radiography we are limited to finding the line integrals of the attenuation coefficients at each point. These represent the product of the attenuation coefficient and the path length over the entire path. If the projection is taken at different energies, using appropriate processing, separate images can be generated representing the line integral, or path lengths, of different materials in the object being studied. A system of this type is described in U.S. Patent 3,848,130 by A. Macovski entitled, "Selective Material X-Ray Imaging System." As in the case with computerized tomography, this patent failed to recognize that the linear attenuation coefficient can be decomposed into a photoelectric component and a Compton-scattering component based on two spectral measurements. This provides a simpler, two-component decomposition of the projection information.